The state of the art of inkjet printing technologies is relatively well developed. A wide variety of inkjet printing devices are available for commercial purchase from consumer desktop printers that produce general documents to commercial wide format printers that produce huge photographic quality posters. Hewlett-Packard Company of Palo Alto Calif., for example, has been particularly active in the development of thermal inkjet printing devices.
A thermal inkjet printer typically comprises a transitionally reciprocating printhead that is fed by a source of ink to produce an image-wise pattern upon some type of receiver. Such printheads are comprised of an array of nozzles through which droplets of ink are ejected by the rapid heating of a volume of ink that resides in a chamber behind a given nozzle. This heating is accomplished through the use of a heater resistor that is positioned within the print head in the vicinity of the nozzle. The heater resistor is driven by an electrical pulse that creates a precise vapor bubble that expands with time to eject a droplet of ink from the nozzle. After the drop is ejected and the electrical pulse is terminated, the ink chamber refills and is ready to further eject additional droplets when the heater resistor is again energized.
The quality of an ejected droplet from a thermal inkjet printhead is dependent upon the precision of the vapor bubble that is produced by the heater resistor, and is therefore also dependent upon how evenly the heater resistor produces heat. Since it is also desirable to shape heater resistors to better control the quality of the ejected droplet, physical characteristics such as current crowding become an issue. Since electrical current will always follow the shortest path, current will crowd and produce more heat in the shorter path when there is both a shorter and a longer path for the current to flow within a particular structure.
U.S. Pat. No. 6,367,147 issued to Giere et al. teaches that multiple heater resistors that are disposed at various angles to one another require coupling devices to connect the resistors and thus turn the current from one heater resistor to another. Since these coupling devices incorporate both short and long paths in which current will flow, the coupling devices must incorporate a compensation resistor to correct the flow of current in a manner that will force the current to flow evenly within the coupling device. In Giere et al., a segmented heater resistor includes a first heater resistor segment and a second heater resistor segment. The coupling device provides serial coupling from the first resistor segment to the second resistor segment with the compensation resistance reducing current crowding within the coupling device.
Heater resistors that are connected in various series and parallel combinations are also subject to the current crowding effect, unless they provide equal paths for the flow of current. In the case where the current paths are not equal, some form of coupler must afford any change of angle of one resistor to another. In this case, the coupler will exhibit uneven heating through the current crowding effect, and compensation resistance within the coupler must be employed. The use of compensation resistors is complicated, costly and expensive. Additionally they produce a voltage drop within the coupler, causing drive voltage inefficiencies. The present invention is directed to overcoming one or more of the problems set forth above.